[asshanson]

If you have fuel pressure at the rail I don't see how that can be an issue. Can you log fuel pressure while you're driving? Even when my CTS-V had fuel issues, basically half the fuel was bleeding back into the tank and not even making it to the feed line, I could do an easy 30% throttle on E85 on a single Walbro 255 and never drop below 60psi. You would have to be starving almost all of the fuel for you to have part throttle lean issues. Either that or letting 100% of the fuel by the regulator back into the tank, but then you wouldn't be showing any fuel pressure.

Log fuel pressure while cruising. If you have just a gauge on the rail with a 1/8" NPT fitting, get a 6' section of hose with NPT fittings on the end and run the gauge into the car so you can read it. This is what I did to verify my fuel pressure was dropping under high throttle. If it is dropping, it could be the main pump getting very weak, or the regulator giving up and letting everything by.